Parasitic nematodes sicken or debilitate millions of personsworldwide. In the majority of these infections, the third larval stage (L3i) infects the vertebrate host. L3i are transitional, developmental^ arrested stages, which are reactivated when exposed to cues present in the definitive host. Mechanisms by which parasites regulate L3i development remain unclear, due largely to the lack of modern molecular biological methods for these organisms. By contrast, systems regulating L3 morphogenesis in the free-living nematode Caenorhabditis elegans are well characterized. These systems include an insulin/IGF signal transduction pathway. The overall goal of the proposed study is to ascertain whether a recently discovered insulin-like pathway in Strongyloides stercoralis also regulates development in that parasite and, by analogy, in parasitic nematodes generally. S. stercoralis was chosen as a model because this worm has an alternate free-living cycle, reminiscent of continuous development in C. elegans. The specific aims of this proposal are, first, to identify key genes encoding insulin-like signal transduction elements in Strongyloides stercoralis. We will complete our characterization of the PI3 kinase-encoding gene pik-1, an ortholog of C. elegans age-1, and seek the S. stercoralis ortholog of daf-28, the insulin-like ligand regulating L3 development in C. elegans. Second, we will investigate the developmental function of insulin-like signal pathway intermediates from S. stercoralis. Methods for these studies will stress heterologous gene transfer into strains of C. elegans carrying specific insulin pathway mutations. Third, we will apply these same methods to investigate the roles of insulin pathway intermediates in regulating lifespan in S. stercoralis. Finally, we will delineate parasite- specific structure/function relationships in FKTF-1 and describe intracellular trafficking of this transcription factor and daf-16 ortholog during development. Chimeric gene constructs, combining functional domains of FKTF-1 and DAF-16 will be expressed in mutant C. elegans and in S. stercoralis and effects on development and subcellular localization assessed. Subcellular localization of FKTF-1 will be determined at key points in the parasite life cycle and compared to that of DAF-16 at analogous points in C. elegans' development. Our recent success with transgenesis in S. stercoralis will allow us to augment experiments using C. elegans as a genetic surrogate with studies of homologous transgene constructs in S. stercoralis itself.